Secondary batteries are rechargeable and dischargeable by using an electrode material having excellent reversibility, and lithium secondary batteries have been commercialized representatively. Lithium secondary batteries are expected to be provided in moveable units such as vehicles or to be applied as medium and large sized power source used in a power storage of a power supply network such as a smart grid, as well as small sized power source of small information technology (IT) appliances such as smart phones, portable computers, and electronic paper.
When lithium metal is used as n negative material of a lithium secondary battery, dendrites may be formed, and thereby causing shorting of the battery or a risk of explosion. Thus, instead of using the lithium metal, crystalline carbon such as graphite and artificial graphite or carbon based active material such as soft carbon or hard carbon capable of intercalating and deintercalating lithium ions has been mainly used as a negative. However, as applications of secondary batteries have increased, demands for secondary batteries having high capacity and high output have increased more, and accordingly, non-carbon based negative materials capable of generating an alloy with lithium, for example, silicon (Si), tin (Sn), or aluminum (Al) having a capacity of 500 mAh/g or greater that may replace the carbon based negative material having a theoretical capacity of 372 mAh/g, have drawn attention.
Among the above non-carbon based negative materials, silicon has a theoretical capacity of about 4200 mAh/g that is the largest among those materials, and thus, applications of silicon are considered to be important in view of capacity. However, since silicon expands about four times greater in volume during a charging operation than during a discharging operation, an electric connection between active materials may break or an active material may be isolated from a current collector due to a volume variation during charging and discharging processes, and an irreversible reaction such as forming of a solid electrolyte interface (SEI) layer such as Li2O may occur and lifespan may degrade because of an erosion of the active material due to an electrolyte. Therefore, there is a barrier in commercializing the silicon as the negative material.
There have been suggested many kinds of techniques for implementing a battery having a relatively high capacity, while minimizing expansion and contraction in a volume of an active material and improving lifespan, and among those, an active material obtained by generating nanosilicon by using SiOx as a matrix is highly possible to be commercialized. An active material using SiOx material as a matrix has lifespan and capacity that have been improved to some degree, but there are actual limitations in using the above active material because of a large irreversible capacity of SiOx. As another approach, there is provided a method of generating nano-sized silicon particles. However, even nano-sized silicon particles have not reached a level of practical use yet, due to damage to particles caused by volume expansion/contraction and rapid degradation of lifespan even though a degree of expansion/contraction is smaller. In addition, reduced size of silicon particles results in reduction in capacity, and thus, there is a limitation in using the silicon particles. Therefore, in order to commercialize the silicon material, it is necessary to restrain the volume variation during the charging and discharging and to maximize a capacity of a battery.